1. Technical Field of the Invention
This present invention relates to a laser beam steering system operable with no moving parts and functional at room temperature.
2. Background Art
The advantages of deflecting and steering optical signals is well known. There are numerous products and industries employing reflected, deflected and steered laser beams for applications such as scanners, laser printer, and laser etching, and in general to the telecommunications and medical industry. Routing laser signals is also useful for a variety of military applications such as counter-measures, range finding, multi-target designation and guidance.
Devices for directing an optical beam or the spatial patterns of illumination produced by lasers have generally been restricted to mechanical methods, such as galvanic mirrors and various mirro/gimbal combinations. The prior art includes many mechanical mirror scanning, acousto-optic and electro-optic methods and apparatus for steering of laser beams, and, the disadvantages of the mechanical methods being bulky, slow, expensive, and relatively inaccurate, and highly susceptible to vibrations and misalignment.
Spatial light modulators are devices having optical properties of the material that are spatially controlled. These light modulator are typically very large compared to the wavelength of light, and have therefore been impractical for obtaining diffraction patterns. Advances in semiconductor technology allow the use of quantum well devices to make smaller spatial light modulators where diffraction effects dominate. These quantum well devices have fast response times, and can be made lithographically using standard fabrication equipment. There has been on-going efforts to create beam steering devices using modulators to eliminate the mechanical devices of the prior art. The optical modulators devices can work in transmission mode, where the light passes directly through the quantum well region, or in reflection mode, where external or integrated mirrors are used to enhance reflectivity changes and contrast.
The prior art has addressed the aspects of light altering the complex refractive index of a semiconductor material, wherein the intensity of an optical wave changes the complex refractive index of Si, GaAs, InGaAsP and other semiconductors in the microwave range (1 mm-1 cm) and infrared (IR) range (1.0 .mu.-100 .mu.). Light induced modulation of both the real and imaginary parts of the refractive index occurs, wherein the real part controls phase and the imaginary part controls amplitude of the modulated electromagnetic field. The real part is primarily responsible for changes in IR waves and the imaginary part for changes in millimeter waves (MMW). This effect is described by Drude theory and involves carrier induced changes in the complex permittivity of metals and semiconductors when illuminated by light.
Under this principle, devices that alter the phase of lightwaves by illumination of semiconductors with a second control lightwave have been developed. In particular, various forms of optical phase modulators have been developed. For example, optically controlled spatial light modulators based on semiconductor materials have been demonstrated. In such an spatial light modulators, optically induced changes in the semiconductor material affect an adjacent layer of electromaterial which in turn affects an EM wave propagating through the electro-optic material.
While spatial light modulators devices transmit some two-dimensional patterns through an optical wave, phased array antennas transmit EM waves in a particular direction in the microwave region without moving parts. A phased array is a network of radiating elements, each of which is usually non-directive but whose cooperative radiation pattern is a highly directed beam because constructive interference occurs between radiating elements. Whereas previous radar antennas had to be mechanically steered for beampointing, the phased array antenna achieves the same effect electronically by individually changing the phases of the signals radiating from each element. Narrow angular band beams can be formed by simply driving each element of the array with an appropriately phased signal. Moreover, electronic steering is much faster and more agile than mechanical beam steering and can form several beam lobes and nulls to facilitate multiple target tracking or other functions such as anti-jamming. The flexibility of electronic steering afforded by phased array radars comes at the cost of individual control of each element. The N elements of the antenna are driven with the same signal but each with a different phase. In practice, a single signal is equally split into N signals to feed the elements, and a phase shifting network, such as those using ferrites or diodes, is provided for individual phase control of each element. For large arrays (i.e., N greater than 100), the complexity of the power splitting network and the cost of providing N phase shifters can become quite high, not to mention the bulkiness of the necessary waveguide plumbing. Moreover, for very large arrays, the computation required to calculate the array phase distribution for a desired radiation pattern is a serious burden.
Phased array antenna theory is based on Fourier optics in general and the theory of diffraction gratings in particular. It is well known from Fourier optics that the optical beam is diffracted in a particular direction if the phase difference between the particular optical rays is a multiple of the wavelength of the optical beam.
With respect to phased array radars, photonic architectures are typically characterized as either optically coherent or non-coherent. Optically coherent architectures are considered impractical because of the thousands of optical signals that must be phase locked.
Military Applications include targeting/guidance and countermeasures systems. The countermeasures system seeks to protect military personnel and property from hostile threats such as missiles, aircraft and helicopters that employ laser tracking systems such as precision guided munitions and other electro-optical guided munitions. For example, a missile with a sensor uses acquisition electronics to home in on a target, such as F22 or F16 aircraft. The aircraft usually has sensors for countermeasures and knows of the missile tracking the aircraft. Thus, missile sees aircraft and aircraft sees missile. The primary function of the aircraft countermeasures system is to blind or distort the missile sensor.
The missile sensor typically operates in the 3-5 micron wavelength range, and the countermeasures systems tries to blind the missile by scanning a pattern with a signal intended to overload or saturate the missile electronics. The aircraft uses a countermeasures laser that operates in the 3-5 micron range and this laser shoots a laser beam at the missile in an attempt to blind the tracking sensor. It is not enough to have a static laser beam that strikes only a portion of the missile, and the countermeasures laser must be scanning or rastering all around the missile threat.
Prior art systems use a camera or other sensor to track the in-coming target and the countermeasure laser mechanically illuminates and tracks the missile using a mirror and gimbal technique. The camera and gimbal are computer controlled in an attempt to focus the laser on the missile by continuously adjusting the mirror by the gimbal. This mirror and gimbal combination is not highly reliable and subject to numerous errors and malfunctions. The stabilization and maintenance of a line of sight on the threat is exceedingly difficult with such a system especially in conditions of excessive vibrations and air turbulence. There are considerable alignment problems with the mirror and gimbal system that make the system inoperable if out of alignment. The gimbal is typically a hydraulic system and the system relies upon the gimbal being able to quickly respond to commands to track the movement of the incoming missile threat.
In addition to being unreliable, the prior art system has additional disadvantages in terms of weight and size. The gimbal system occupies valuable real estate aboard military aircraft and is fairly heavy. Furthermore, the mirror gimbal combination is relatively expensive.
A prior art approach to laser guided targeting system is disclosed in U.S. Pat. No. 4,737,028. This prior art system comprises a sensor system capable of tracking both the target in one band and the laser designator in another band. This patent describes the prior art steering mirror control with the camera processing.
In U.S. Pat. No. 5,222,071, a dynamic optical grating device is described using a semiconductor device to steer laser beams. This invention involves interband electron flow between two bands.
U.S. Pat. No. 5,305,123 describes a light controlled spatial and angular electromagnetic wave modulator. Periodic perturbations of the dielectric field in the surface of a semiconductor material are induced by an optical control pattern, which causes electromagnetic waves to be coupled out of the semiconducting material in a particular direction depending upon the period of the perturbations. Rapid variations in the period of the perturbations are induced by controlling the optical control pattern. By rapidly changing the period of the perturbations, the device can be used to control the direction of the reflected/transmitted beam and beam steering and forming can be achieved.
What is needed is a system for electronically steering laser beams that do not involve mechanical gimbals or external mirrors. In order to be practical, such a system should operate at room temperature without requiring thermal conditioning. The beam steering system should be easily fabricated using reliable and inexpensive methodologies, with flexibility to allow the designer various options and customization. The system should allow both transmission and reflection of the light signals depending upon the application and
The invention is devised in the light of the problems of the prior art described herein. Accordingly it is a general object of the present invention to provide a novel and useful apparatus and technique that can solve the problems described herein.
An object of the present invention is an integrated circuit or chip that does the same function of the gimbal/mirror combination but without the aforementioned problems. The present invention rasters the laser beam without using mechanical means. A voltage or other electrical parameter is changed by sweeping the voltage through the chip to scan the laser beam without a mirror or gimbal.
The present device operates as a mirror in reflection mode and an optical medium in transmitter mode. In the reflection mode, the laser beam is reflected by the chip in a manner to steer the reflected laser output beam by varying the voltage of the chip. This is the functional equivalent of the mirror, with no moving parts. Alternatively the present invention also allows a transmitting function operable with an optical medium, wherein the transmitted laser beam through the other side of the chip is steered according to the applied voltage. Such a configuration is not possible with the classic mirror designs.
Different schemes exist in the prior art, however the present invention provides significant benefits and is distinguishable. For example, U.S. Pat. No. 5,222,071 describes a dynamic grating device that uses applied voltage for beam steering. However, the ""071 invention performs all electron distribution using interband transmissions, whereas the present invention accomplishes the electron distribution in a single band. Such a single band electron distribution is termed intraband or intersubband transitions. The implementation of the intraband electron distribution is a better implementation and provides cleaner transitions.
In one embodiment the present invention is manufactured or built as an optical modulator. For example, in a chip with a whole number of devices, a 256xc3x97256 array with a 15 micron pitch connected in a massively parallel fashion using hybridization technology to interconnect to the drive circuitry. The size of the array may vary depending upon the specific application and circumstances.
An object of the invention is a laser beam steering device to steer an infrared laser beam, comprising at least one optical absorption modulator formed on a substrate and having a quantum well doped with electrons. The laser beam being incident on the modulator, wherein discrete voltage signals applied to the modulator control an exit angle of one or more exit beams from the modulator, and wherein the modulator operates using intersubband transitions in the quantum wells.
A further object includes the optical beam steering device, wherein the substrate is an application specific integrated circuit (ASIC) with integrated drive circuitry electrically coupled to the modulator. It also includes the optical beam steering device, wherein the modulator is formed as a one dimensional array or two dimensional array of modulators.
In addition, the optical beam steering device, wherein the modulators form a transmission array wherein the laser beam travels through the transmission array. Alternatively, the optical beam steering device, wherein the at least one modulator forms a reflection array wherein the laser beam is at least partially reflected from the reflection array.
Another object is the optical beam steering device, further comprising a microcontroller and/or a memory section coupled to the modulator for processing the voltage signals for applying voltage patterns.
An object of the invention is an electrically steerable laser system for countermeasures, comprising an input laser signal, an array of optical absorption modulators, at least one input optical device coupling the laser signal to the optical modulators, an electronic control section for producing a voltage pattern to the modulators, wherein the voltage pattern operates in an intersubband transition to steer the input laser signal and produce a steered laser beam, and at least one output optical device coupled to the array and outputting the steered laser beam.
An object also includes an all optical switching system, comprising an input fiber optic
bundle having input optic signals;
an array of optical absorption modulators;
at least one input optical device coupling said input optic signals to said optical modulators;
an electronic control section for producing a voltage pattern to said modulators, wherein said voltage pattern operates in an intersubband transition to steer said input optic signals and produce a steered optical output;
an output fiber optic bundle having output optic signals; and
at least one output optical device coupled to said array and said output fiber optic bundle.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.